Vitrified grinding stone and method of manufacturing the same

ABSTRACT

A vitrified grinding stone includes an abrasive grain and a vitrified binder. The vitrified grinding stone has a porosity, a degree of concentration of abrasive grains and an abrasive grain diameter according to a preset processing efficiency and a processing precision of grinding.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims priority under 35 U.S.C. § 119 to Japanese Patent Application No. 2003-155149, filed May 30, 2003, entitled “VITRIFIED GRINDING STONE AND A METHOD OF MANUFACTURING THE SAME”. The contents of this application are incorporated herein by reference in their entirety. The present application is a continuation-in part of PCT/JP2004/007754 which was filed May 28, 2004. The contents of PCT/JP2004/007754 are incorporated herein by reference in their entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a vitrified grinding stone and a method of manufacturing the vitrified grinding stone.

2. Discussion of the Background

Since the degree of binding and composition of vitrified grinding stones are readily adjusted and they afford resistance to water, alkali, and oil, they are widely employed in grinding and polishing operations, including precision grinding.

For example, in small-diameter internal grinding, in which the inner surfaces of small-diameter nozzles such as engine injection nozzles are ground, the peripheral speed of the grinding stone is limited and the quill rigidity becomes low. Thus, in order to maintain good grinding, it is required to make the diameter of the grinding stone as large as possible. For such reasons, grinding stones having diameters close to those of the inner diameter of the object being processed are employed in small-diameter internal grinding. However, in such grinding, the chip length increases and clogging tends to occur. These tendencies are particularly marked when the grinding efficiency is increased.

FIGS. 2A-2C show the relation between the effective cutting edge spacing of the grinding stone and the chip pocket in internal grinding. To prevent clogging and increase processing efficiency during grinding, the method (FIG. 2B) of increasing the abrasive grain diameter and increasing the size of the effective cutting edge spacing We and chip pocket P relative to the norm (FIG. 2A) is conceivable. However, in this method, since the effective cutting edge spacing We is broadened, the processing precision (surface roughness) ends up decreasing. On the other hand, to increase processing precision, the method of reducing the abrasive grain diameter and decreasing the size of the effective cutting edge spacing We and chip pocket P relative to the norm (FIG. 2A) is also conceivable (FIG. 2C). However, in this method, since the volume of chip pocket P is reduced, the chip pocket ends up quickly filling with chips of the ground products. When grinding is further continued in this state, the grinding stone clogs, causing fusion. Due to this tradeoff between the improvement of the processing efficiency and that of processing precision of grinding, it has been difficult to simultaneously achieve both.

Conventionally, attempts have been made to simultaneously achieve processing efficiency and processing precision of grinding. For example, the method of decreasing the degree of concentration of abrasive grains given by the ratio of abrasive grains present in the grinding stone to increase processing efficiency and processing precision of grinding has been conceived. However, when the degree of concentration of abrasive grains is reduced, the binding property between abrasive grains decreases, causing a problem in the binding of abrasive grains. Further, the dispersion of abrasive grains decreases, causing a problem that abrasive grains cannot be uniformly dispersed in the grinding stone. Thus, in methods of reducing the degree of concentration of the abrasive grains, it has been difficult to achieve both processing efficiency and processing precision of grinding.

The method of replacing a portion of the cBN abrasive grains with a hollow inorganic substance is also known. Japanese Unexamined Patent Publication (KOKAI) Showa No. 62-251077 discloses such method. The contents of this application are incorporated herein by reference in their entirety. In this method, a hollow inorganic substance is pulverized during grinding to form pores, which would be expected to produce an effect similar to that of chip pockets. However, in grinding stones in which a portion of the cBN abrasive grains is replaced with a hollow inorganic substance and the degree of concentration is made about 100, dispersion of the abrasive grains deteriorates due to the decreased degree of concentration, making it difficult to obtain grinding stones in which the abrasive grains are uniformly dispersed. Further, since the hollow inorganic substance is also held by a vitrified binder, the vitrified binder that originally should have held the cBN abrasive grains ends up being trapped in the hollow inorganic substance. Thus, when employing a hollow inorganic substance, it becomes necessary to use more vitrified binder than usual, resulting in a drawback in the form of decreased porosity. Further, when the hollow inorganic substance is damaged during grinding, the vitrified binder used to bind the hollow inorganic substance ends up remaining in the grinding stone, resulting in a drawback in the form of impaired grinding.

Methods of forming pores by employing organic pore-forming materials such as walnuts, wood chips and the like are known. The pore-forming materials are incorporated into a molded product prior to calcination and burned out during calcination, yielding pores in the grinding stone obtained after calcination. The use of such a pore-forming material is desirable in that it does not have the drawbacks encountered when fillers such as a hollow inorganic material are incorporated into the grinding stone and permits the achievement of a low degree of concentration.

However, depending on the type of pore-forming material employed, there is a drawback in that shrinking tends to occur to a greater degree than in common grinding stones in which common alumina-based (A-based) abrassive grains employing WA abrasive grains (white alumina abrasive grains) and the like are used because the abrasive grains are not oxides and a large quantity of vitrified binder is employed. There are further drawbacks in that the use of conventional pore-forming materials makes it hard to uniformly disperse the pores and is unsuited to vitrified cBN grinding stones in which more uniform distribution of abrasive grains is required than in common grinding stones.

In recent years, for the purpose of improving conventional pore-forming materials, methods of reducing shrinkage due to calcination and uniformly dispersing pores even when manufacturing vitrified grinding stones in which the degree of concentration of abrasive grains is made less than 200 have been known. For example, Japanese Unexamined Patent Publication (KOKAI) No. 2000-317844 discloses such method. The contents of this application are incorporated herein by reference in their entirety. The grinding stones obtained by such methods afford advantages in that, even when the degree of concentration is small, less than 200, the grinding ratio is high, grinding burns and fusion tend not to occur, and it is easy to use.

However, even in the aforementioned method, when the degree of concentration is further lowered, that is, when pore-forming materials having a diameter greater than the average grain diameter of the abrasive grains are employed, the spacing between individual abrasive grains widens and thus the effective cutting edge spacing increases. As a result, there is a problem in that good processing precision cannot be maintained. The above-cited method also presents the problem of inadequate uniformity of pores and abrasive grains in the grinding stone. Accordingly, there is a need for further improvement in the above-cited method for maintaining good processing efficiency and processing precision even in grinding stones in which the degree of concentration has been further reduced.

SUMMARY OF THE INVENTION

According to one aspect of the present invention, a vitrified grinding stone includes an abrasive grain and a vitrified binder. The vitrified grinding stone has a porosity, a degree of concentration of abrasive grains and an abrasive grain diameter according to a preset processing efficiency and a processing precision of grinding.

According to another aspect of the present invention, a vitrified grinding stone includes an abrasive grain and a vitrified binder. A ratio of pores having a size 1 to 3 times an average grain diameter of the abrasive grain in a volume of whole pores ranges from 20 to 70 volume percent.

According to yet another aspect of the present invention, a vitrified grinding stone includes an abrasive grain and a vitrified binder. A a ratio of pores having a size 0.1 to 1 time an average grain diameter of the abrasive grain in a volume of whole pores ranges from 30 to 70 volume percent.

According to further aspect of the present invention, a method of manufacturing a vitrified grinding stone which has an abrasive grain and a vitrified binder includes setting a processing efficiency and a processing precision of grinding, and setting a porosity, a degree of concentration of abrasive grains and an abrasive grain diameter according to the processing efficiency and the processing precision.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete appreciation of the invention and many of the attendant advantages thereof will be readily obtained as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings, wherein:

FIG. 1 is an enlarged schematic cross-sectional view of the vitrified grinding stone according to an embodiment of the present invention;

FIGS. 2A-2C are drawings showing the relation between the effective cutting edge spacing We and the chip pocket P in a grinding stone during internal grinding;

FIG. 3 is an enlarged schematic cross-sectional view showing the structure of a grinding stone manufactured using a conventional burnout material;

FIG. 4 is an enlarged schematic explanatory drawing showing the structure of a grinding stone manufactured without using a burnout material;

FIG. 5 is an explanatory drawing showing the relation between the grinding efficiency ratio and the effective cutting edge spacing in the grinding stone according to an embodiment of the present invention and a conventional grinding stone;

FIG. 6 is an explanatory drawing descriptive of the arrangement of abrasive grains and the effective cutting edge spacing in an embodiment of the present invention;

FIG. 7 is a drawing showing the grinding efficiency ratio when the effective cutting edge spacing is 0.1 mm in an embodiment of the present invention and a comparative example;

FIGS. 8A to 8C show the results (power consumption, surface roughness, and abrasion) when internal grinding was conducted at a grinding processing efficiency of 0.3 mm³/(mm·sec) with the grinding stones of Examples 1 to 3 and Comparative Example 2; and

FIGS. 9A to 9C show the results (power consumption, surface roughness, and abrasion) when internal grinding was conducted at a grinding processing efficiency of 0.7 mm³/(mm·sec) with the grinding stones of Examples 1 and 2.

DESCRIPTION OF THE EMBODIMENTS

The embodiments will now be described with reference to the accompanying drawings, wherein like reference numerals designate corresponding or identical elements throughout the various drawings.

The present inventors conducted extensive research into the relation between the effective cutting edge spacing of abrasive grains and chip pocket volume for the purpose of achieving both processing efficiency and processing precision of grinding. As a result, they discovered that presetting the processing efficiency and processing precision of grinding and then setting the porosity, degree of concentration of abrasive grains, and abrasive grain diameter based on the preset processing efficiency and processing precision of grinding resulted in a method permitting improvement in both grinding efficiency and processing surface roughness. The vitrified grinding stone according to the embodiment of the present invention and manufacturing method thereof will be described in greater detail below.

[Vitrified Grinding Stone]

<Processing Efficiency and Processing Precision of Grinding>

The grinding stone according to the embodiment of the present invention has a porosity, a degree of concentration of abrasive grains, and an abrasive grain diameter based on the preset processing efficiency and processing precision of grinding. The processing efficiency of grinding is given as the amount of grinding per second for a grinding stone width of 1 mm and is normally denoted in units of mm³/(mm·sec). The processing precision of grinding can be denoted as a surface roughness and is normally denoted as a ten-point average roughness Rz (μm).

For example, in internal grinding, when attempting to achieve a processing precision of grinding of less than or equal to 1 Rz (μm), the conventional limit of processing efficiency of grinding is about 0.3 mm³/(mm·sec). By contrast, in the grinding stone according to the embodiment of the present invention, it is possible to achieve a processing efficiency of grinding of greater than or equal to 0.3 mm³/(mm·sec) even at a grinding precision of less than or equal to 1 Rz (μm). More specifically, even when the processing precision of grinding is set to 0.1 to 1.6 Rz (μm), preferably 0.2 to 1.0 Rz (μm), and more preferably 0.3 to 0.5 Rz (μm), the processing efficiency of grinding can be set to 0.1 to 2.0 mm³/(mm·sec), preferably 0.2 to 1.0 mm³/(mm·sec), and more preferably 0.3 to 0.7 mm³/(mm·sec).

Here, the relation between the grinding efficiency ratio and the effective cutting edge spacing, We, is shown in FIG. 5 to describe the relation between the above-mentioned processing efficiency of grinding and the effective cutting edge spacing, We. As shown in FIG. 5, for example, when a conventional grinding stone has an effective cutting edge spacing of 0.1 mm, the grinding efficiency ratio is less than 2. By contrast, when the grinding stone according to the embodiment of the present invention has an effective cutting edge spacing of 0.1 mm, it is possible to make the grinding efficiency ratio greater than or equal to 2 (preferably greater than or equal to 2.5, and more preferably greater than or equal to 3.0). (FIG. 5 shows an example where the grinding efficiency ratio is greater than or equal to 3 when the effective cutting edge spacing is 0.1 mm.) In the grinding stone according to the embodiment of the present invention abrasive grains of prescribed size (preferably 10 to 90 μm) are selected, and the abrasive grains are not positioned next to one another in the manner of a conventional grinding stone (see FIG. 2), but are arranged uniformly as shown in FIG. 6, maintaining a certain effective cutting edge spacing. Thus, the grinding stone according to the embodiment of the present invention achieves good processing efficiency (grinding efficiency ratio) while maintaining a prescribed processing precision of grinding.

<Porosity>

In the present Specification, the term “porosity” means the ratio of the volume of pores (space) without abrasive grains, binder, and other fillers and the like, to the volume of the whole grinding stone. In the present invention, the porosity is comprised of a forced porosity and a natural porosity. Here, the term “forced porosity” means the ratio of the volume of burnout pores—formed by burning out a pore-forming material when a molded product containing at least an abrasive grain, a vitrified binder, and a pore-forming material is calcined in a calcination step—to the volume of whole pores. The term “natural porosity” refers to the porosity calculated by subtracting the above forced porosity from the total porosity, and is the ratio occupied in the molded product of gap portions in the abrasive grain, vitrified binder, and pore-forming material prior to calcination.

In the vitrified grinding stone according to the embodiment of the present invention, the porosity suitably falls within a range of 30 to 70 volume percent, preferably 40 to 60 volume percent, and more preferably 45 to 55 volume percent, of the volume of the whole grinding stone. When the porosity is greater than or equal to 30 volume percent, fusion is not caused due to inadequate volume of chip pockets and clogging during grinding. Since the pore-forming material is burned out during the calcination in the embodiment of the present invention, better porosity can be ensured than in grinding stones in which a pore-forming material is not employed; porosities of up to 70 volume percent can be obtained.

Within this porosity, the forced porosity suitably falls within a range of 5 to 35 volume percent, preferably 20 to 35 volume percent, and more preferably 25 to 35 volume percent, of the volume of the whole grinding stone. In the vitrified grinding stone according to the embodiment of the present invention, the forced pores formed by the pore-forming material primarily contribute to the improvement of the processing efficiency of grinding. When the forced porosity is greater than or equal to 5 volume percent, grinding can be carried out well. When the forced porosity is less than or equal to 35 volume percent, grinding stones can be manufactured stably.

In the vitrified grinding stone according to the embodiment of the present invention, the size of the forced pores formed by burning out the pore-forming material greatly affects grinding stone performance. For example, the smaller the forced pores, the greater the dispersion of abrasive grains and pores in the grinding stone. Since increasing dispersion of the abrasive grains and the pores stabilizes the cutting edge spacing, the chip discharge performance increases and power consumption during grinding decreases, which are advantageous with regard to production efficiency. Further, since the strength of the grinding stone increases, abrasion of the grinding stone due to grinding decreases, resulting in good durability. The vitrified grinding stone according to the embodiment of the present invention can be one comprising pores (including both forced pores and natural pores) having a size 1 to 3 times the average grain diameter of the abrasive grains in a ratio of 20 to 70 volume percent, preferably 30 to 60 volume percent, and more preferably 30 to 50 volume percent, with respect to the volume of whole pores. The vitrified grinding stone according to the embodiment of the present invention can be one comprising pores having a size 0.1 to 1 time the average grain diameter of the abrasive grains in a ratio of 30 to 70 volume percent, preferably 40 to 70 volume percent, and more preferably 50 to 70 volume percent, with respect to the volume of whole pores. The ratio of pores having a desired size can be adjusted by suitably setting the size and quantity added of the pore-forming material employed. The ratio of pores having a desired size can be calculated by slicing the grinding stone, measuring the cross-section with a microscope capable of three-dimensional measurement to obtain three-dimensional data, and then analyzing the cross-sectional shape.

<Pore-Forming Material>

The pore-forming material employed in the embodiment of the present invention is not specifically limited, other than that it be a material that can be burned out in calcination. It is preferable to use a pore-forming material having a burnout starting temperature greater than or equal to the transition temperature of the vitrified binder described further below, and having a burnout ending temperature lower than the maximum temperature within the calcination temperature range of the vitrified binder.

For example, it is suitable to use a pore-forming material having a burnout starting temperature at least 5° C. (more preferably at least 10° C., and further preferably at least 20° C.) greater than the transition temperature of the vitrified binder, and having a burnout ending temperature at least 5° C. (more preferably at least 10° C., and further preferably at least 20° C.) lower than the maximum temperature within the calcinations temperature range of the grinding stone starting materials including the vitrified binder.

The pore-forming material desirably has a strength so as to preclude pulverization during stirring of the manufacturing starting materials in the process of manufacturing the grinding stone. Any pore-forming material having a strength so as to preclude pulverization during stirring may be employed, whether it be solid or hollow.

The specific gravity of the pore-forming material is desirably greater than or equal to 1 (for example, 1 to 2.5, preferably 1 to 1.5). When the specific gravity of the pore-forming material is greater than or equal to 1, it does not float on the starting materials during stirring and can be uniformly dispersed in the starting materials.

The size of the pore-forming material is preferably selected according to the size of the desired forced pores. As set forth above, the smaller the forced pores, the lower the power consumption during grinding and the greater the advantage afforded in the form of production efficiency. Further, the smaller the forced pores, the greater the strength of the grinding stone and the less the abrasion of the grinding stone during grinding, resulting in good durability. However, when the forced pore diameter becomes excessively small, the processing efficiency during grinding drops. From the above perspectives, the size of the pore-forming material is suitable from 0.1 to 3 times the average grain diameter of the abrasive grains. In particular, from the perspective of power consumption during grinding and grinding stone durability, the size of the pore-forming material is preferably from 0.16 to 1 time the average grain diameter of the abrasive grains. For example, when employing cBN abrasive grains as an abrasive grain, when the average grain diameter of the abrasive grains is 22 to 36 μm, a pore-forming material about 3.5 to 36 μm in size can be employed.

The shape of the pore-forming material is not specifically limited. However, an abrasive grain having a true spherical shape that can be dispersed well during the manufacturing process is preferred.

The content, as volume percentage, of the pore-forming material in the starting materials is preferably 10 to 50 percent, more preferably 15 to 45 percent, and further preferably 15 to 40 percent. When the volume percentage is greater than or equal to 10 percent, an effect by the formation of burnout pores can be achieved. When the volume percentage is less than or equal to 50 percent, a grinding stone of suitable strength and durability can be manufactured.

Specific examples of pore-forming materials are: polymer compounds such as polymethyl acrylate and polymethyl methacrylate, and carbonaceous compounds containing 90 mass percent or more of carbon. The use of polymethyl methacrylate as a pore-forming material is preferred.

<Abrasive Grains>

The grain diameter of the abrasive grain employed in the embodiment of the present invention can be suitably determined in view of the relation between the porosity and the degree of concentration based on the above-described processing efficiency and processing precision of grinding. For example, within the above-stated ranges of grinding efficiency and processing precision of grinding, it is suitable to use abrasive grains having an average grain diameter ranging from 10 to 90 μm, preferably 18 to 60 μm, more preferably 20 to 55 μm, and most preferably 25 to 45 μm. With abrasive grains having an average grain diameter of greater than or equal to 10 μm, there is no problem with adhesion between abrasive grains and processing efficiency of grinding does not drop sharply. With abrasive grains having an average grain diameter of less than or equal to 90 μm, a prescribed cutting edge spacing can be maintained and processing precision can be improved.

The type of abrasive grain is not specifically limited other than that the average grain diameter falls within the above-stated range. For example, cBN abrasive grains, A-based (alumina-based), and C-based (silicon carbide-based) abrasive grains can be employed. When grinding the inner surface of a high-precision component, cBN abrasive grains are preferably employed. One type of abrasive grain may be employed alone, or two or more types may be mixed for use.

When employing cBN abrasive grains as an abrasive grain, one or more types of common abrasive grains and hollow inorganic materials may be employed as a filler as needed. However, in that case, the quantity of filler employed is suitably adjusted so that the degree of concentration of the cBN abrasive grains ranges from 50 to 160.

Further, when employing diamond abrasive grains as an abrasive grain, it is desirable to suitably set the types of vitrified binder and pore-forming material and manufacturing conditions such as the calcination temperature to prevent deterioration of the diamond abrasive grains.

The degree of concentration of the abrasive grains is suitably from 50 to 160, preferably from 75 to 150, and more preferably from 100 to 125. Here, the term “degree of concentration” means the ratio of abrasive grains in the grinding stone. For example, in the case of diamond abrasive grains, 4.4 ct/cm³ is the degree of concentration of 100 corresponding to 25 volume percent. Accordingly, the degree of concentration of 200 corresponds to 50 volume percent. When abrasive grains having a different density from diamond abrasive grains are employed, the difference in density from diamond abrasive grains is taken into account and the degree of concentration is established in accordance with the above. When the abrasive grains are cBN abrasive grains, in the same manner as diamond abrasive grains, the degree of concentration of 100 corresponds to about 25 volume percent and the degree of concentration of 200 to about 50 volume percent.

In the embodiment of the present invention, the degree of concentration is adjusted within a relatively low range of 50 to 160 as well as the porosity is adjusted within a range of 30 to 70 volume percent, as mentioned above, to maintain or increase a prescribed chip pocket volume and prevent clogging and fusion of the grinding stone during high-efficiency grinding.

<Vitrified Binder>

In the embodiment of the present invention, the vitrified binder can be suitably selected based on the type of abrasive grain. For example, when manufacturing a vitrified cBN grinding stone employing cBN abrasive grains as an abrasive grain, the vitrified binder can be, for example, borosilicate glass or crystallized glass. An example of crystallized glass is one from which willemite has been precipitated. To achieve adequate holding strength, the coefficient of thermal expansion of the vitrified binder desirably falls within a range of ±2×10⁻⁶ (1/K) (room temperature to 500° C.) with respect to the coefficient of thermal expansion of the abrasive grains.

When employing a vitrified binder for superabrasive grains as a vitrified binder, the temperature for calcining grinding stone starting materials containing binder is selected based on the type of the vitrified binder for superabrasive grains employed. Since the transition temperature of the vitrified binder for superabrasive grains is lower than the transition temperature of vitrified binders for common abrasive grains, the temperature of calcining grinding stone starting materials containing vitrified binder for superabrasive grains preferably falls within a range of 650 to 1,000° C., more preferably within a range of 700 to 950° C. At greater than or equal to 650° C., a grinding stone having a certain strength even after calcination is obtained. At less than or equal to 1,000° C., the superabrasive grains do not deteriorate.

An example of a preferred composition of the vitrified binder for superabrasive grains is SiO₂:40 to 70 mass percent, Al₂O₃:10 to 20 mass percent, B₂O₃:10 to 20 mass percent, M¹O:2 to 10 mass percent, and M² ₂O:2 to 10 weight percent. Here, M¹ denotes one or more metals selected from alkaline earth metals, and M² denotes one or more metals selected from alkali metals.

The content of vitrified binder can be suitably selected. For example, the content thereof may fall within a range of 13 to 35 volume percent, preferably within a range of 18 to 22 volume percent, with respect to the volume of the starting materials.

In the vitrified grinding stone according to the embodiment of the present invention, it suffices for at least the portion contributing to grinding to have the above-stated composition. Accordingly, the vitrified grinding stone according to the embodiment of the present invention includes, for example, those in which a vitrified grinding stone portion containing abrasive grains and vitrified binder is provided on a support surface made of ceramic not containing abrasive grains.

Further, when the grinding stone according to the embodiment of the present invention is a vitrified superabrasive grain grinding stone, the additives normally employed in vitrified superabrasive grain grinding stones, such as embrittling agents and solid lubricants, can be incorporated in suitable quantity as desired.

The vitrified grinding stone according to the embodiment of the present invention may be wheel-shaped, cylinder-shaped, rectangle-shaped and the like.

[Method of Manufacturing Vitrified Grinding Stone]

The method of manufacturing vitrified grinding stones according to the embodiment of the present invention will be described in greater detail below.

The manufacturing method according to the embodiment of the present invention comprises steps of setting a processing efficiency and a processing precision of grinding, and setting a porosity, a degree of concentration of abrasive grains and an abrasive grain diameter based on the processing efficiency and processing precision. As regards the processing efficiency and processing precision of grinding, porosity, degree of concentration of abrasive grains, and abrasive grain diameter, those regarding the above-described vitrified grinding stone may be employed without alteration. Further, the abrasive grains, vitrified binder, and pore-forming material employed in the vitrified grinding stone according to the embodiment of the present invention set forth above may be suitably employed as the abrasive grains, vitrified binder, and pore-forming material in the manufacturing method according to the embodiment of the present invention.

The manufacturing method according to the embodiment of the present invention may comprise a calcinations step in which a molded product containing at least an abrasive grain, vitrified binder, and a pore-forming material is calcined to burn out the pore-forming material. In the manufacturing method according to the embodiment of the present invention, the method of calcining a molded product containing at least an abrasive grain, vitrified binder, and a pore-forming material is preferably one in which the molded product is calcined by maintaining it at a certain temperature for a certain period to burn out the pore-forming material. Such a method is preferable in that the pore-forming material burns out before the vitrified binder melts in the calcination step, preventing calcination shrinkage and disruption of the abrasive grain distribution caused by the binder and abrasive grains moving about freely.

The period of maintaining mentioned above is preferably long enough for the aforementioned pore-forming material contained in the molded product to burn out. A period adequate for the pore-forming material to burn out can be suitably set based on the shape and dimensions of the grinding stone being manufactured.

When maintaining the aforementioned molded product at the calcination temperature of the vitrified binder, it is maintained at a certain temperature falling within the range of the calcination temperature. So long as the temperature remains within this calcination temperature range, variation in the temperature (for example, a rise in temperature over time) is permissible.

The temperature that is maintained for a certain period during calcination is preferably greater than or equal to the burnout ending temperature of the pore-forming material (preferably a temperature at least 5° C. greater than the burnout ending temperature, more preferably a temperature at least 10° C. greater than the burnout ending temperature). The temperature of calcining the molded product (maximum temperature during calcination) can be a temperature within the calcination temperature range of the vitrified binder as well as higher than or equal to the burnout ending temperature of the pore-forming material.

In the manufacturing method according to the embodiment of the present invention, the dimension of the molded product in the course of calcining the molded product is preferably a dimension so as to permit adequate burnout of the pore-forming material employed. For example, in the case of a molded product in the form of a rectangular parallelepiped, the thickness (the dimension in which the rectangular parallelepiped is the thinnest) can be set to less than or equal to 10 mm (preferably less than or equal to 5 mm, more preferably less than or equal to 3 mm). As a further example, when the molded product is in the shape of a cylinder, the edge thickness (the thickness of the cylinder wall) can be made less than or equal to 10 mm (preferably less than or equal to 5 mm, more preferably less than or equal to 3 mm).

In the manufacturing method according to the embodiment of the present invention, the atmosphere during calcination is one in which the pore-forming material burns adequately. When the pore-forming material is carbonaceous, an atmosphere containing oxygen can be employed with air normally being adequate.

In the manufacturing method according to the embodiment of the present invention, the step yielding the molded product can be inserted before the calcination step.

The molded product is preferably obtained by mixing and stirring starting materials comprising at least abrasive grains, a vitrified binder powder, and a pore-forming material with a primary binder such as an adhesive paste to obtain a mixture in which each of the components has been uniformly dispersed, and molding this mixture by pressing and drying.

When manufacturing a vitrified superabrasive grain grinding stone, desired additives such as embrittling agents, solid lubricants, and molding adjuvants that are commonly employed in vitrified superabrasive grain grinding stones may be incorporated into the above starting materials in suitable quantity.

The vitrified grinding stone obtained by the above manufacturing method can be employed as a grinding stone in various grinding devices. Even when the diameter of the object being ground is small, high processing efficiency and processing precision of grinding are achieved. Thus, it is suited to use in internal grinding. Examples of applications of the grinding stone of according to the embodiment the present invention include grinding of the inner surfaces and sheet surfaces of the injection nozzles of fuel injection devices and pressure regulators, and internal grinding of the inner and outer wheels of bearings.

EXAMPLES

Examples according to the embodiment of the present invention will specifically described below. Vitrified grinding stones of Examples and Comparative Examples described below are wheel-shaped ones.

Suitable modification of the materials, quantities employed, ratios, processing contents, and processing sequences described in Examples is possible without departing from the spirit of the present invention. Accordingly, the scope of the present invention must not be restrictively interpreted to the specific examples below.

1. Manufacturing of Grinding Stone and Structure Thereof.

Starting materials of the following blend shown in Examples 1 to 3 and Comparative Examples 1 and 2 were press molded and calcined in air for 24 hours at 900° C. (during which they were maintained at 900° C. for one hour) to obtain vitrified grinding stones. In Example 1, when the decrease in mass was measured under the condition of raising a temperature of 10° C./min, the burnout starting temperature (a reduction of 10 mass percent) of polymethyl methacrylate was found to be 300° C. and the burnout ending temperature (a reduction of 90 mass percent) was found to be 500° C. The transition temperature of the vitrified binder employed was 550° C. and the specific calcination temperature was 850 to 950° C.s. <Starting materials of Example 1 and blend thereof> cBN abrasive grain 55.1 volume parts (average grain diameter: 30 μm (#600), degree of concentration: 160) Polymethyl methacrylate 17.4 volume parts (average grain diameter: 30 μm, true specific gravity: 1.2) Vitrified binder 27.5 volume parts Adhesive paste 14.5 volume parts

<Structure of grinding stone of Example 1 after calcination> cBN abrasive grain 40.0 volume parts Pore 40.0 volume parts Burnout pore (forced pore): 10.0 volume parts Natural pore: 30.0 volume parts Ratio of pores having a size 1 to 3 times the average grain diameter of abrasive grain: 37 volume percent Vitrified binder 20.0 volume parts

<Starting materials of Example 2 and blend thereof> cBN abrasive grain 55.1 volume parts (average grain diameter: 30 μm (#600), degree of concentration: 160) Polymethyl methacrylate 17.4 volume parts (average grain diameter: 5 μm, true specific gravity: 1.2) Vitrified binder 27.5 volume parts Adhesive paste 14.5 volume parts

<Structure of grinding stone of Example 2 after calcination> cBN abrasive grain 40.0 volume parts Pore 40.0 volume parts Burnout pore (forced pore): 10.0 volume parts Natural pore: 30.0 volume parts Ratio of pores having a size 0.1 to 1 time the average grain diameter of abrasive grain: 67 volume percent Vitrified binder 20.0 volume parts

<Starting materials of Example 3 and blend thereof> cBN abrasive grain 56.5 volume parts (average grain diameter: 30 μm (#600), degree of concentration: 160) Polymethyl methacrylate 21.0 volume parts (average grain diameter: 5 μm, true specific gravity: 1.2) Vitrified binder 22.5 volume parts Adhesive paste 14.5 volume parts

<Structure of grinding stone of Example 3 after calcination> cBN abrasive grain 40.0 volume parts Pore 40.0 volume parts Burnout pore (forced pore): 14.0 volume parts Natural pore: 30.0 volume parts Vitrified binder 16.0 volume parts

<Starting materials of Comparative Example 1 and blend thereof> cBN abrasive grain 55.1 volume parts (average grain diameter: 30 μm (#600), degree of concentration: 160) Carbonaceous beads (150 μm) 17.4 volume parts Vitrified binder 27.5 volume parts Adhesive paste 14.5 volume parts

<Structure of grinding stone of Comparative Example 1 after calcination> cBN abrasive grain 43.7 volume parts Pore 40.0 volume parts Burnout pore (forced pore): 10.0 volume parts Natural pore: 30.0 volume parts Vitrified binder 16.3 volume parts

<Starting materials of Comparative Example 2 and blend thereof> cBN abrasive grain 69.2 volume parts (average grain diameter: 30 μm, degree of concentration: 180) Vitrified binder 30.8 volume parts Adhesive paste 14.3 volume parts

<Structure of grinding stone of Comparative Example 2 after calcination> cBN abrasive grain 45.0 volume parts Pore (natural pore) 35.0 volume parts Vitrified binder 20.0 volume parts

FIGS. 1, 3 and 4 are enlarged schematic cross-sectional views of the structures of the grinding stones of Example 1 and Comparative Examples 1 and 2 obtained after calcination. As shown in FIG. 1, the grinding stone according to the embodiment of the present invention is a grinding stone in which cBN abrasive grains 1 are bonded by vitrified binder 3, and burnout pores (forced pores) 2 and natural pores 4 are present. As shown in FIG. 3, the grinding stone of Comparative Example 1 is a grinding stone in which cBN abrasive grains 21 and burnout pores 22 are bonded by vitrified binder 23, and pores 24 are present. As shown in FIG. 4, the grinding stone of Comparative Example 2 is a grinding stone in which cBN abrasive grains 31 are bonded by vitrified binder 32, and pores 33 are present.

When the structure of the grinding stone according to the embodiment of the present invention is compared to those of the grinding stones of Comparative Examples, the grinding stone of Example 1 shown in FIG. 1 has more uniformly dispersed abrasive grains and pores and greater porosity than the grinding stones of Comparative Examples 1 and 2. By contrast, the grinding stone of Comparative Example 1 shown in FIG. 3, despite having good porosity, has nonuniformly dispersed abrasive grains. The grinding stone of Comparative Example 2 shown in FIG. 4 has nonuniform abrasive grains and low porosity. This reveals that the grinding stone according to the embodiment of the present invention is a grinding stone having good chip pocket size while maintaining a certain effective cutting edge spacing.

2. Evaluation of Vitrified Grinding Stone (1)

The grinding stones obtained in Example 1 and Comparative Examples 1 and 2 were used to conduct internal grinding and the relation between grinding efficiency ratio and the size of the effective cutting edge spacing was examined. FIG. 7 gives the results. The ground objects, the processing conditions, and the dressing conditions are given below. <Ground object> Material SCM415 Dimension Internal diameter φ 3.95 mm Grinding allowance φ0.05 mm

<Processing condition> Machine employed Grinder for internal grinding Grinding type Wet oscillation grinding Peripheral speed of grinding stone 22.6 m/s Peripheral speed of ground object 0.5 m/s Grinding efficiency ratio 1-3.2 Oscillation Done Grinding oil Oil-based

<Dressing condition> Dresser φ50 square column rotary Dress depth of cut φ 1 μm/pass Lead 0.004 mm/rev

In FIG. 7, for an identical effective cutting edge spacing We (0.1 mm), it was possible to conduct normal grinding to a grinding efficiency ratio up to 3.2 in Example 1. By contrast, it was only possible to conduct normal grinding to a grinding efficiency ratio up to 1.9 in Comparative Examples 1 and 2. This reveals that for an identical processing precision of grinding, the vitrified grinding stone according to the embodiment of the present invention affords a processing efficiency of grinding of about 1.7 times that of conventional grinding stones.

3. Evaluation of Vitrified Grinding Stone (2)

Internal grinding was conducted at a processing efficiency of grinding of 0.3 mm³/(mm sec) with the grinding stones obtained in Examples 1 to 3 and Comparative Example 2, and the power consumption, surface roughness, and abrasion were examined. The change in power consumption is shown in FIG. 8A, the results of surface roughness measurement are shown in FIG. 8B, and the results of abrasion measurement are shown in FIG. 8C. The grinding stones obtained in Examples 1 and 2 were used to conduct internal grinding at a grinding efficiency of 0.7 mm³/(mm sec), and the power consumption, surface roughness, and abrasion were examined. The change in power consumption is shown in FIG. 9A, the results of surface roughness measurement are shown in FIG. 9B, and the results of abrasion measurement are shown in FIG. 9C. However, the grinding stone of Comparative Example 2 underwent fusion during processing immediately after dressing, precluding subsequent evaluations. <Ground object> Material SUJ-2 Dimension Internal diameter φ 28.3 mm Grinding allowance φ0.36 mm

<Processing condition> Machine employed Grinder for internal grinding Grinding type Wet oscillation grinding Peripheral speed of grinding stone 45 m/s Peripheral speed of ground object 1.25 m/s Oscillation Done Grinding oil Water-soluble

<Dressing condition> Dresser φ25 square column rotary Dress depth of cut φ 4 μm/pass Lead 0.030 mm/rev (i) Power Consumption

As shown in FIG. 8A, the grinding stone of Comparative Example 2 exhibited extremely high power consumption during initial grinding and fused, precluding subsequent grinding. By contrast, as shown in FIGS. 8A and 9A, the grinding stones of Examples 1 to 3 exhibited low levels of power consumption that were maintained stably during grinding without fusion, permitting continuous grinding.

(ii) Surface Roughness

As shown in FIG. 8B, the grinding stones of Examples 1 to 3 achieved a processing precision of grinding of less than or equal to 0.7 Rz (μm) at a processing efficiency of grinding of 0.3 mm³/(mm·sec).

Further, as shown in FIG. 9B, the grinding stones of Examples 1 and 2 achieved a processing precision of grinding of less than or equal to 0.8 Rz (μm) at a processing efficiency of grinding of 0.7 mm³/(mm·sec).

(iii) Abrasion

Comparing Examples 1 and 2 as shown in FIGS. 8C and 9C, the grinding stone of Example 2, which had a pore-forming material of smaller diameter (that is, smaller forced pores), had greater strength and thus underwent less abrasion. Comparing Examples 2 and 3, in which a pore-forming material of identical diameter was employed, as shown in FIG. 8C, the grinding stone of Example 2, which contained more binder, was harder and thus underwent less abrasion.

As set forth above, the vitrified grinding stone according to the embodiment of the present invention has a porosity, degree of concentration of abrasive grains, and abrasive grain diameter that are based on preset processing efficiency and processing precision of grinding. Thus, the grinding stone according to the embodiment of the present invention affords precision processing of roughness of surfaces being processed while improving processing efficiency of grinding, formerly considered to be contradicting indicators of grinding stones.

In the method of manufacturing a vitrified grinding stone according to the embodiment of the present invention, the processing efficiency and processing precision of grinding are preset. Based on the processing efficiency and processing precision of grinding, the porosity, degree of concentration of abrasive grains, and abrasive grain diameter are set. Thus, the method of manufacturing according to the embodiment of the present invention permits uniform distribution of abrasive grains and pores within the grinding stone, thereby permitting the manufacturing of grinding stones affording both processing efficiency and processing precision of grinding.

According to the embodiments of the present invention, a vitrified grinding stone in which a prescribed porosity is maintained and the pores and abrasive grains are uniformly disposed even when small-diameter abrasive grains are employed is provided.

According to the embodiments of the present invention, a method of manufacturing vitrified grinding stones capable of maintaining a prescribed porosity in the grinding stone and achieving uniform dispersion of abrasive grains and pores in the grinding stone even when small-diameter abrasive grains are employed is provided.

In the embodiments of the present invention, the processing efficiency and processing precision of grinding are preset. Thus, the embodiments of the present invention can provide both a vitrified grinding stone having a porosity, degree of concentration of abrasive grains, and abrasive grain diameter based on the aforementioned processing efficiency and processing precision of grinding, and a method of manufacturing the same. For example, the embodiments of the present invention can provide a grinding stone having good processing precision of less than or equal to 1.0 Rz (μm) even at a processing efficiency of grinding of greater than or equal to 0.3 mm³/(mm·sec). Further, the embodiments of the present invention can provide a grinding stone in which abrasive grains and pores are uniformly dispersed and the degree of concentration is maintained at 50 to 160 with a porosity of 30 to 70 volume percent by containing a forced porosity of 5 to 35 volume percent based on burnout pores even when small-diameter abrasive grains having an average grain diameter of 10 to 90 μm are employed. As a result, the embodiments of the present invention affords a uniform cutting edge spacing comparable to that of large-diameter abrasive grains even when employing small-diameter abrasive grains, and maintains the chip pocket volume. Thus, it provides both a vitrified grinding stone in which clogging tends not to occur during grinding, fusion is prevented, and both grinding processing efficiency and grinding processing precision are achieved, and a method of manufacturing the same.

Obviously, numerous modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described herein. 

1. A vitrified grinding stone comprising: an abrasive grain; and a vitrified binder, wherein the vitrified grinding stone has a porosity, a degree of concentration of abrasive grains and an abrasive grain diameter according to a preset processing efficiency and a processing precision of grinding.
 2. The vitrified grinding stone according to claim 1, wherein when said processing precision of grinding ranges from 0.1 to 1.6 Rz (μm), the processing efficiency of grinding ranges from 0.1 to 2.0 mm³/(mm·sec).
 3. The vitrified grinding stone according to claim 1, wherein said porosity ranges from 30 to 70 volume percent with respect to a volume of the whole grinding stone.
 4. The vitrified grinding stone according to claim 1, wherein said porosity comprises a forced porosity based on burnout pores formed by burning out a pore-forming material.
 5. The vitrified grinding stone according to claim 4, wherein said forced porosity ranges from 5 to 35 volume percent with respect to a volume of the whole grinding stone.
 6. The vitrified grinding stone according to claim 4, wherein said pore-forming material has a size 0.1 to 3 times the average grain diameter of the abrasive grain.
 7. The vitrified grinding stone according to any of claims 1, wherein a ratio of pores having a size 1 to 3 times an average grain diameter of the abrasive grain in a volume of whole pores ranges from 20 to 70 volume percent.
 8. The vitrified grinding stone according to claim 1, wherein a ratio of pores having a size 0.1 to 1 time an average grain diameter of the abrasive grain in a volume of whole pores ranges from 30 to 70 volume percent.
 9. The vitrified grinding stone according to claim 4, wherein said pore-forming material is a polymer compound.
 10. The vitrified grinding stone according to of claim 1, wherein said abrasive grain has an average grain diameter ranging from 10 to 90 μm.
 11. The vitrified grinding stone according to claim 1, wherein said degree of concentration of abrasive grains ranges from 50 to
 160. 12. The vitrified grinding stone according to claim 1, wherein said abrasive grain is a cubic boron nitride abrasive grain.
 13. A vitrified grinding stone comprising: an abrasive grain; and a vitrified binder, wherein a ratio of pores having a size 1 to 3 times an average grain diameter of the abrasive grain in a volume of whole pores ranges from 20 to 70 volume percent.
 14. A vitrified grinding stone comprising: an abrasive grain; and a vitrified binder, wherein a ratio of pores having a size 0.1 to 1 time an average grain diameter of the abrasive grain in a volume of whole pores ranges from 30 to 70 volume percent.
 15. The vitrified grinding stone according to claim 13, wherein said abrasive grain has an average grain diameter ranging from 10 to 90 μm.
 16. The vitrified grinding stone according to claim 13, wherein the degree of concentration of said abrasive grains ranges from 50 to
 160. 17. A method of manufacturing a vitrified grinding stone comprising an abrasive grain and a vitrified binder, the method comprising: setting a processing efficiency and a processing precision of grinding; and setting a porosity, a degree of concentration of abrasive grains and an abrasive grain diameter according to the processing efficiency and the processing precision.
 18. The method of manufacturing according to claim 17, wherein said processing precision of grinding is set within a range of 0.1 to 1.6 Rz (μm) and said processing efficiency of grinding is set within a range of 0.1 to 2.0 mm³/(mm·sec).
 19. The method of manufacturing according to claim 17, wherein said porosity is set within a range of 30 to 70 volume percent with respect to a whole volume of the grinding stone.
 20. The method of manufacturing according to claim 17, wherein said porosity comprises a forced porosity based on burnout pores formed by burning out a pore-forming material.
 21. The method of manufacturing according to claim 20, wherein said forced porosity is set within a range of 5 to 35 volume percent with respect to a whole volume of the grinding stone.
 22. The method of manufacturing according to claim 20, wherein a pore-forming material having a size 0.1 to 3 times an average grain diameter of the abrasive grain is employed as said pore-forming material.
 23. The method of manufacturing according to claim 17, wherein a polymer compound is employed as said pore-forming material.
 24. The method of manufacturing according to claim 17, wherein an abrasive grain having an average grain diameter ranging from 10 to 90 μm is employed as said abrasive grain.
 25. The method of manufacturing according to claim 17, wherein said degree of concentration of abrasive grains is set within a range of 50 to
 160. 26. The method of manufacturing according to claim 17, wherein a cubic boron nitride abrasive grain is employed as said abrasive grain.
 27. The vitrified grinding stone according to claim 14, wherein said abrasive grain has an average grain diameter ranging from 10 to 90 μm.
 28. The vitrified grinding stone according to claim 2, wherein said porosity ranges from 30 to 70 volume percent with respect to a volume of the whole grinding stone. 